The proposed studies form part of a continuing investigation of the cellular mechanisms underlying the transport of water, ions, and water soluble non-electrolytes across the epithelial cell layer of the small intestine. Specifically the project seeks to focus on the kinetics, metabolic dependence and electrophysiological correlates of ionic movements across this tissue. Coupling between the transport of ions (particularly sodium) and that of certain organic solutes (e.g. sugars and amino acids) and the effects of coupling on the electrophysiological behavior of small intestinal epithelia is of major concern within the overall scope of the investigation. Experimental techniques employed include the following measurements: transepithelial potential difference and short circuit current, membrane potentials of epithelial cells, intracellular ionic activities (using ion selective microelectrodes), and transepithelial fluxes (using radiotracers). Because of its exceptional stability in vitro and its close functional resemblance to mammalian small intestine, the isolated small intestine of the bullfrog is being used as an experimental system. The goal of this investigation is to provide information which, it is hoped, will help to form a basis for the development of new conceptual and technical approaches to the control and management of human disorders which stem from intestinal malfunction. In addition, the techniques employed and the results obtained in this study will, hopefully, contribute significantly to the broader problems of epithelial physiology and pathophysiology in general.